@@ -176,7 +176,7 @@ return res.newVal
 
 # Validators
 
-proc validate*(value: MinValue, t: string): bool =
+proc validate*(i: In, value: MinValue, t: string): bool =
   case t:
     of "bool":
       return value.isBool
@@ -201,16 +201,31 @@ return value.isString
     of "a":
       return true
     else:
-      var split = t.split(":")
-      # Typed dictionaries 
-      if split[0] == "dict":
-        if value.isTypedDictionary(split[1]):
-          return true
-      return false
+      if t.contains(":"):
+        var split = t.split(":")
+        # Typed dictionaries 
+        if split[0] == "dict":
+          if value.isTypedDictionary(split[1]):
+            return true
+        return false
+      elif i.scope.hasSymbol("type:$#" % t):
+        # Custom type class
+        var i2 = i.copy(i.filename)
+        i2.withScope():
+          i2.push value
+          i2.pushSym("type:$#" % t)
+          let res = i2.pop
+          if not res.isBool:
+            raiseInvalid("Type class '$#' does not evaluate to a boolean value ($# was returned instead)" % [t, $res])
+          return res.boolVal
+      else:
+        raiseInvalid("Unknown type class '$#'" % t)
 
-proc validType*(s: string): bool =
+proc validType*(i: In, s: string): bool =
   const ts = ["bool", "null", "int", "num", "float", "quot", "dict", "'sym", "sym", "string", "a"]
   if ts.contains(s):
+    return true
+  if i.scope.hasSymbol("type:$#" % s):
     return true
   for tt in s.split("|"):
     if not ts.contains(tt) or tt.startsWith("dict:"):
@@ -236,12 +251,12 @@ var split = element.split("|")
     if split.len > 1:
       var res = false
       for t in split:
-        if validate(value, t):
+        if i.validate(value, t):
           res = true
           break
       if not res:
         raiseInvalid(message(value.typeName))
-    elif not validate(value, element):
+    elif not i.validate(value, element):
       raiseInvalid(message(value.typeName))
     valid.add element
 

@@ -202,7 +202,7 @@ check = c mod 2 != 0
       if check:
         if v == "==>":
           o = true
-        elif not validType(v):
+        elif not i.validType(v):
           raiseInvalid("Invalid type specified in signature at position $#" % $(c+1))
         else:
           if o:
@@ -258,11 +258,11 @@ var r = false;
           if o.contains("|"):
             let types = o.split("|")
             for ut in types:
-              if validate(x, ut):
+              if i.validate(x, ut):
                 r = true
                 break
           else:
-            r = validate(x, o)
+            r = i.validate(x, o)
           if not r:
             discard i.pop
             raiseInvalid("Invalid value for output symbol '$#'. Expected $#, found $#" % [outVars[k], o, $x])
@@ -493,6 +493,16 @@ 
   def.symbol("type") do (i: In):
     let vals = i.expect("a")
     i.push vals[0].typeName.newVal
+
+  def.symbol("typeclass") do (i: In):
+    let vals = i.expect("'sym", "quot")
+    let name = vals[0].getString
+    let symbol = "type:$#" % name
+    let code = vals[1]
+    info "[typeclass] $1 = $2" % [symbol, $code]
+    if i.scope.symbols.hasKey(symbol) and i.scope.symbols[symbol].sealed:
+      raiseUndefined("Attempting to redefine sealed typeclass '$1'" % [name])
+    i.scope.symbols[symbol] = MinOperator(kind: minValOp, val: code, sealed: false, quotation: true)
 
   def.symbol("import") do (i: In):
     var vals = i.expect("'sym")

@@ -212,6 +212,12 @@ if i.pop.kind == minQuotation:
       i.push true.newVal
     else:
       i.push false.newVal
+
+  def.symbol("stringlike?") do (i: In):
+    if i.pop.isStringLike:
+      i.push true.newVal
+    else:
+      i.push false.newVal
   
   def.symbol("dictionary?") do (i: In):
     if i.pop.isDictionary:

@@ -3,3 +3,5 @@ * Added **sealed?** and **sealed-sigil?** symbols.
 + **floor** and **ceil** now correctly return an integer again.
 * Added **abs** symbol.
 * Now executing min shell if no file is specified (unless input is piped in), without the need of specifying **-i**.
+* Implemented the possibility to define type classes.
+* Added **stringlike?** symbol.

@@ -264,6 +264,22 @@ ) ::
     nt"float"
   ) assert
 
+  (:n ((n integer?) (n 0 >)) &&) 'natural typeclass
+  ("type:natural" defined?) assert
+  (
+    symbol natural-sum
+    (natural :n natural :m ==> natural :result)
+    (n m + @result)
+  ) ::
+  null :err
+  (
+    (3 -3 natural-sum)
+    (@err)
+  ) try
+  pop ;Remove 3 that was left on the stack.
+  (err format-error "expected: natural natural natural-sum" match) assert
+  (2 3 natural-sum 5 ==) assert
+
   report
   ; Tidy up
   clear-stack

@@ -132,6 +132,10 @@ (true quotation? false ==) assert
   (false quotation? false ==) assert
   (("a" 2 c) quotation?) assert
 
+  ("a" stringlike?) assert
+  (1 stringlike? false ==) assert
+  ('test stringlike?) assert
+
   ({} 'module type? false ==) assert
   ((1 2 3) 'module type? false ==) assert
   (4 'module type? false ==) assert